1. Field of the Invention
The present invention relates to a color printer which color-decomposes an original image and forms a color image on a recording medium in accordance with the color-decomposed image signals.
2. Description of the Prior Art
Prior art electrographic color printers use several systems, such as those described below.
In one system, an original image is sequentially color-decomposed to three colors, latent images are formed one for each of the color-decomposing filters, the latent images are developed by complementary color developers and the developed images of different colors are superimposed to reproduce a color image.
This system can attain fast reproduction and is inexpensive but improvement of the quality of the image is limited because the system cannot attain .gamma.-correction, multi-tone reproduction and monochromatic image reproduction.
In another system, three-color-decomposed image data are stored in a frame memory, the contents of the memory are sequentially read after image processing, a laser beam, for example, is used as an electrographic recording light source and it is modulated by the image signals, latent images are formed and they are developed by complementary color developers and the developed images are superimposed to reproduce a color image.
In this system, because of the image processing, the quality of graphic image is improved over the first system but the system is very expensive because of the large capacity of the frame memory. For example, assuming that an original image on a size A4 sheet is to be stored with a picture cell size of 0.15 mm square, the memory capacity needed for four colors is (210 mm.times.297 mm)/(0.15 mm.times.0.15 mm).times.4 (sheets).times.8 (bits)=88,704,400 bits. Assuming that the cost per bit in future is 0.01, it costs about 890,000. This is the cost for only the memory elements, and when an assembly and adjustment cost and the printed circuit board cost are added, the total cost will be approximately 1,000,000, which is very expensive.
However, in the prior art system, such a frame memory is essential for other reasons than the image processing as described below.
(A) When multi-color superposition printing is effected with a single photosensitive drum, the four color image-processed data must have been stored in the frame memory before the printing starts, because the recording station has only one-color memory and hence the three-color decomposed image data of the input image and the monochromatic data must be stored in the frame memory.
(B) In a system having a plurality of, for example, four photosensitive drums and four recording stations, recording start positions on the photosensitive drums are adjusted when the images are transferred to a print paper in order to register the positions of the respective color images. Thus, the frame memory is necessary to adjust the recording start positions for the respective color photosensitive drums.